PROJECT SUMMARY/ABSTRACT Rapid weight gain during infancy is an important target for obesity prevention efforts and formula-/bottle- feeding has been highlighted as one of the strongest modifiable predictors of risk for rapid weight gain during the postpartum period. Current recommendations and prevention programs aimed at reducing risk for rapid weight gain and obesity during early childhood have predominately focused on promoting responsive feeding practices, regardless of milk type or feeding mode used. However, the evidence-base for how to promote responsive feeding during bottle-feeding is scant, primarily due to methodological limitations of previous research. Within-subject studies that allow for examination of mothers' feeding practices during both breast and bottle-feeding are needed to elucidate the most effective targets for tailored primary prevention efforts. The aim of the proposed research is to address current research gaps by conducting a within-subject, laboratory-based experimental study of 40 mother-infant dyads wherein we will observe dyads during both bottle- and breastfeeding interactions. Strengths of this approach will be the ability to: 1) isolate effects of feeding mode on feeding outcomes; 2) consider how individual differences in maternal sensitivity to infant cues moderates effects of feeding mode on feeding outcomes; and 3) control for unmeasured covariates that may influence both feeding decisions and practices. To this end, we will objectively assess infant intake during these two modes of feeding by weighing the baby before and after the feeding. We will also video-record feeding observations to allow for behavioral coding of maternal sensitivity to infant cues and infant clarity of cues using the Nursing Child Assessment Parent-Child Interaction Feeding Scale (NCAFS), and will place electrocardiogram (ECG) leads on both mothers and infants to assess behavioral and physiological synchrony during feeding interactions. These data will be used to address the following specific aims: Aim 1: To examine whether maternal sensitivity to infant cues moderates effects of feeding mode on infant intake; Aim 2: To identify key predictors of low maternal sensitivity to infant cues; Exploratory Aim: To examine whether maternal-infant behavioral and physiological synchrony differs between bottle-feeding versus breastfeeding. This innovative study will provide novel insights related to how feeding mode and mothers' feeding practices interact to influence infant feeding outcomes and will create an important foundation for further research related to the primary prevention of obesity during early infancy.